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This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University London Heat pipes have recently been introduced as thermal absorbers for photovoltaic panels, with the objective of increasing the performance of Photovoltaic/Thermal (PV/T) technologies, which simultaneously produce electrical and thermal energy. To best fit surface cooling applications, advances in the heat pipe designs have been witnessed with the recent introduction of multi-channel flat heat pipes as efficient heat transfer mediums between photovoltaic cells and heat sink. Despite the promising experimental results observed, the complex two-phase heat transfer mechanisms taking place in multi-channel flat heat pipes are poorly understood and remain to be investigated. In addition to the lack of theory and analytical models considering the flat shape and multi-channel internal geometry, numerical modelling of heat pipes using computational fluid dynamic (CFD) technics is still at an early stage. In this regard, this study investigates thoroughly the two-phase heat transfer in a novel multi-channel flat heat pipe using three approaches: theoretical, numerical, and experimental. The main objectives of this research are as follows: 1) Provide a better understanding of two-phase heat transfer in a multi-channel geometry, 2) Develop an analytical model to predict the performance of a multi-channel flat heat pipe which considers the two-phase heat transfer mechanisms taking place in this new geometry, 3) Simulate the working cycle of multi-channel heat pipes using Computational Fluid Dynamics (CFD) techniques, and 4) Compare the developed analytical model and numerical simulations with experimental data. In this thesis, analytical, numerical, and experimental investigations of two-phase heat transfer in a novel multi-channel flat heat pipe are reported. Based on the two-phase heat transfer theory, a novel analytical model was proposed and used in an iterative tool to predict the performance of the multi-channel heat pipe. In addition, several in-house user-defined functions (UDFs) have been developed and tested to simulate the two-phase heat transfer in multi-channel heat pipes using the Lee model. To develop the analytical and numerical models, a unique three leg multi-channel heat pipe was built and tested. In a second phase, the developed models have been used to predict the thermal performance and simulate the two-phase heat transfer in a novel multi-channel flat heat pipe. The models have been compared to experimental findings from the multi-channel flat heat pipe apparatus for validation. The research demonstrated that the analytical model proposed can predict and describe the two-phase heat transfer that allows the novel multi-channel flat heat pipe to be one of the most efficient Photovoltaic/Thermal systems reported up to date. New opportunities for surface cooling applications using the promising multi-channel flat heat pipes are emerging. |
